1. Field of the Invention
The present invention relates to an interference sensor for measuring a physical quantity. It is particularly, but not exclusively, concerned with an interference sensor being an optical gyroscope.
2. Summary of the Prior Art
It is well known to measure a physical quantity using a sensor in which two beams of light, or other radiation, are caused to interfere, with that interference being dependent on the physical quantity to be measured. Thus, optical gyroscopes are known in which two beams of light are caused to propagate in opposite directions through a loop of optical fibre. The two light beams will interfere when they emerge in opposite directions from the loop, and are caused to follow the same optical path. When the loop rotates, the propagation path lengths of the two light beams are changed, so that there is a change in the interference between the light beams. The rotational angular velocity can then be determined on the basis of changes in the phase difference between the interfering light beams.
There are two known systems for measuring the change in the phase difference of the interference. The first known system is known as the "closed loop" type, in which the loop contains additional means for varying the phase of the beams to maintain a constant phase difference therebetween. The detector, which detects the interfering beams, therefore detects a substantially constant phase difference, and the phase difference caused by rotation of the loop of the optical gyroscope is determined from the degree of adjustment needed within the loop itself.
The second system, known as the "open loop" type, directly detects changes in the phase difference between the interfering beams. The present invention is primarily concerned with this latter system.
It is also known to introduce an optical modulator into the optical system of the interference sensor, and to analyse changes in phase on the basis of that modulation. Various modulation systems are known, of which one type is phase modulation. Examples of such phase modulation are disclosed in U.S. Pat. Nos. 4,545,682, 4,634,282, 4,645,345, 4,687,330, 4,707,136, 4,728,192, and 4,765,739, and Electronics Letters, Vol. 19, No. 23, Nov. 10, 1983, pp. 997-999.